SEEDS SCATTERED BY EXPLOSIVE MECHANISMS 87 



collect on the wagon wheels and are carried to the highways or 



to other fields. Threshing machines are important agents in 



scattering seeds, for in their traveling through the country seeds 



of various kinds are jostled 



from them and seed the 



fields and highways. 

 Man scatters many 



weeds by sowing unclean 



seed. Clover seed, Alfalfa 



seed, Grass seed, Wheat, 



Oats, etc., are often ob- 



tained from distant states 



or even from foreign coun- 



tries for seeding. Weed 



seeds are usually present FIG. 89. The three-valved pod of the 



in agricultural seeds, and violet throwing its seeds. Much enlarged. 



sometimes they are pres- After BeaL 



ent in large quantities. In tracing weeds, it has been found 



that many of the most troublesome ones have come from Europe, 



Asia, or some other foreign country. Man has carried the seeds 



and fruits of these weeds across the seas, and most of them have 



been imported and sown with agri- 

 cultural seeds. 



Seeds Scattered by Explosive 

 or Spring-like Mechanisms. In 

 this kind of dissemination the 

 plant itself is the agent which, 

 either by sudden ruptures due to 

 strains or by explosions due to 

 the swelling of certain tissues, is 

 able to throw the seeds often a 

 considerable distance. In the 

 pods of some plants, as in the 

 Vetches, Witch-hazel, Castor 

 FIG. 90. The Squirting Cu- Bean, and Field Sorrel, bands of 



cumber (Ecbalium Elaierium) tissue, which ripen under tension, 



squirting its seeds from the pod. 



podg 



suddenly rupture with so much violence that the seeds are thrown 

 in every direction. In the Violets the carpels, as they ripen and 

 dry, press harder and harder upon the seeds, which suddenly 



